1. Field of the Invention
The present invention relates to a boiler or water heater equipped with a burner, including a housing surrounding a boiler compartment, a cylindrical heat exchanger, which divides the boiler compartment into a combustion chamber, and an exhaust chamber; whereby said heat exchanger comprises passages distributed across its surface for the hot exhaust gas, and a burner head positioned in the combustion chamber.
2. Review of the Prior Art
Such a boiler or water heater is disclosed in the French Patent document No. 93 00498, and is incorporated herein by reference. It contains a number of boiler designs, which exhibit the above-mentioned design features. These boilers are designed to accommodate gas burners and comprise a cylindrical casing with the sides closed off, and a plurality of flame openings distributed across the surface area. Such a gas-fired boiler or water heater is space-saving in its design and does not require a separate furnace room.
It has been a long-standing desire for such a space-saving furnace to be capable of being operated with fuel oil. The disadvantage of using gas as fuel is the more complex fuel storage requirements as compared to oil. As a result, the gas furnace has to rely on either an expensive pressurized tank or on a connection to a gas distribution network. Oil, on the other hand, has been stored in sufficient quantities in tanks located at the site of the furnace in thousands of installations without any problems. Additionally, the supply and filling of the tanks with oil is substantially simpler and less dangerous as compared to gas.
It is therefore the purpose of the invention to provide a furnace, which can be operated by an oil burner without making it larger than a comparable gas furnace. Furthermore, the furnace should be capable to be operated with a gas or an oil burner. Additionally, it is an objective of this invention to provide a furnace characterized by low exhaust emissions, reduced heat loss, and low noise levels.
The objectives are met, in accordance to the intent of this invention, by adding a fire tube comprising an axial flame opening and a flame deflection piece positioned at a distance to the flame opening to the burner head; whereby, the fire tube is designed in such as manner that the flame between the fire tube and the heat exchanger is deflected.
One of the advantages of a furnace designed per this invention is that it can be operated by burners, which produce a lance-shaped flame. Such a flame usually requires a very long combustion chamber. A flame deflection piece, designed in accordance to this invention, allows the length of the combustion chamber to be significantly reduced. The deflection piece turns the flame back to its origin and, as a result, reduces the length of the combustion chamber by xc2xd. This allows the combustion chamber to be almost fully saturated by the flame exiting the fire tube which is subsequently being deflected into the opposite direction. The deflection of the flame to its origin has the further advantage that very hot gas is present around the fire tube very soon after the flame is initiated, enhancing the cold start characteristics of the furnace. Another advantage of deflecting the flame in the above-described manner lies in the fact that the combustion chamber can be utilized much more effectively and, therefore, can be designed in a more compact fashion as compared to those systems that produce a long, thin flame. More specifically, because of the burner head being surrounded by the flame, the entire length of the combustion chamber is more thermally uniform and therefore better-suited to exchange heat energy to the heat exchange medium.
It is advisable for the heat exchanger to comprise a blocking plate, which effectively limits the combustion system in lengthwise direction. In addition to the exhaust chamber, this creates an additional chamber into which exhaust gas flows from the exhaust chamber. The exhaust gas is cooled by the heat exchanger and is partially re-circulated back to the fire tube in order to cool the flame, and partially exhausted through the flue. A preferred blocking plate design separates the discharge chamber from the boiler compartment at its side facing away from the combustion chamber, whereby said discharge chamber is connected to the flue. Such a discharge chamber resides axially inside the boiler. This allows a uniform flow of the exhaust gas from peripheral the areas into the discharge chamber. This avoids non-uniform thermal loading issues of the heat exchanger. It is preferred for the blocking plate to separate a re-circulation chamber from the boiler compartment. Cooled exhaust gas can then re-circulate through this re-circulation chamber into the fire tube in order to cool the flame. The re-circulation chamber can also serve the function of a discharge chamber. It would be preferred, if the discharge chamber and/or re-circulation chamber (separated by the blocking plate) were surrounded by the heat exchanger. This allows additional cooling of the exhaust gas entering these chambers prior to leaving the furnace. As a result of the dual contact with the heat exchanger, the exhaust gas is cooled to approximately 80 degrees C., at continuous operation under full load. This allows for the exhaust gas to be piped directly into a flue (made of a plastic compound) upon exiting the boiler.
Further, it would be preferred that the blocking plate between the combustion chamber and the exhaust discharge chamber is shaped in a curved manner in such a way as to allow an increase in the length of the combustion chamber while minimizing the space requirements of the exhaust discharge chamber. Such a design feature results in a relatively large ratio of the heat exchanger surface surrounding the exhaust discharge chamber with respect to its volume.
In order to reduce the number of required parts, the flame deflection piece should form the blocking plate. In doing so, the position of the deflection piece in relation to the housing wall has certain acoustic advantages. The domed surface should point towards the exhaust discharge chamber. The purpose of this domed surface is to deflect the flame without the participation of any heat exchange elements, permitting the use of the entire heat exchange area since the deflection piece does not obstruct any passages for the hot exhaust gas. A preferred flame deflector design comprises a flame separator, positioned along the centerline of the flame and a ring-shaped deflector dish, which surrounds the flame separator. The flame separator divides the flame, and the deflector dish reflects the flame in order to change the flame direction by 180 degrees. The deflector dish should be made circumferentially uniform in order for the flame to retain a uniform shape after its deflection.
The casing of the heat exchanger consists of pipes positioned adjacent to one another with clearance between the pipes; whereby, said pipes are positioned to surround the combustion chamber and are connected to a supply and a return line. The heat exchange pipes should be wound in a screw-like manner. Such a heat exchanger unit is easy to manufacture; it comprises a large surface area and provides for passages between the pipes. Additionally, pipes can be made to thinner wall thicknesses as compared to castings, and therefore offer a more dynamic heat transfer characteristic which is reflected by higher performance at reduced space requirements. It would also be beneficial if this heat exchanger were assembled from a plurality of heat exchanger units. The individual heat exchanger units have the advantage of using shorter pipe lengths, as compared to a single unit having one long pipe system, resulting in higher through-flow velocities.
The heat exchanger units should be connected to the supply and return in parallel. Heat exchanger units using individual elements disclosed in the French Patent No. 93 00498 are applied successfully. The units described in said documents are characterized by a flat cross-sectional area of the pipes, resulting in an increase in the heat exchange area compared to typical pipes with round cross-sectional areas. Furthermore, an essential advantage of these heat exchanger units is due to the fact that these units are in series production for gas furnaces, and are therefore readily available in the marketplace at excellent quality levels.
The burner should be equipped with exhaust gas re-circulation in order to exceed the current regulated emission values, especially during cold starts.
Even if gas burners are applied to the boiler described by this invention, it is still preferred to use oil burners, simply because oil can be stored in inexpensive tanks, which can be readily refilled. The dependence on a supply distribution network can thus be avoided. Furthermore, the handling of oil is much less hazardous as compared to the handling of gas, which must be stored under pressure in appropriate tanks, in cases where it is not supplied through a gas distribution network.
It would be of benefit, however, to have the capability of the burner to operate on both fuels, oil as well as gas. If the burner head is designed to accommodate oil as well as gas, both fuels can be used in the same furnace alternatively with only minimal effort. This has the advantage that price changes and supply shortages can be dealt with in a more proactive fashion, or in a case of having to wait for a hook-up to be in place to provide gas to the furnace, oil can be used as a temporary means to power the furnace until the gas supply is secured.
For oil operation, an injector sprays oil into the exhaust gas being re-circulated into the fire tube; the inlet openings into the fire tube for fresh air as well as exhaust are designed so that fresh air and exhaust are mixed together inside the hollow cylinder or the hollow truncated cone making up the turbulent zone. As the oil mixes with the exhaust, it fully evaporates prior to its mixing with air. This assures very low exhaust emission values and an excellent starting behavior of the burner.
For gas operation, an air supply passage is used for introducing the gaseous fuel. The inlet openings into the fire tube for the fuel/air mixture and the re-circulated exhaust gas, respectively, are designed so that fuel/fresh air mixture and the exhaust are mixed together inside the hollow cylinder or the hollow truncated cone making up the turbulent zone. Because of these similar methods of introducing the fuels, the same fire tube can be applied to both fuels, gas and oil. It is feasible, to leave the oil injector in place during gas operation and to leave the gas supply system in place during oil operation, in order to provide a dual-fuel furnace with a single burner. A system, such as the one described above, is capable of achieving exhaust emission values of below 60 mg Nox per KW when operating on oil and under 20 mg Nox per KW when operating on gas. The CO values also lie under 20 mg Nox per KW at a very low level. In addition to these very low emission levels, the furnace is capable of performing very well under cold start conditions.
In the combustion chamber, in the area between the fire tube or deflected flame and the heat exchanger, resides a cylindrical combustion chamber shroud, comprising openings for the hot combustion gas. This combustion chamber shroud provides a uniform distribution of the hot combustion gas to the heat exchanger and also serves as an ash collector. It protects the heat exchanger from direct contact with the flame. This allows the distance between the flame and the heat exchanger to be very small. Additionally, this combustion chamber shroud has a positive influence in terms of noise suppression. The openings are arranged so that the combustion gas exits the combustion chamber shroud tangentially in order to facilitate the flow of the gas through the heat exchanger casing to also occur in a tangential direction. This improves the heat transfer as compared to a system that utilizes a radial through-flow direction.
The housing of this unit should be proportioned to be similar in size of a wall heating unit or a xe2x80x9cplug-inxe2x80x9d kitchen unit. The housing, including the air supply line and exhaust line, should have a length of approximately 50 cm. A short design should be able to operate with a boiler length of 30 cm. This means that a separate room for this furnace is not required. It can be stored in a cabinet. The air supply line should surround the exhaust gas line in a counter-flow arrangement in order to pre-heat the incoming air by the warm exhaust gas. The blower should be placed next to the housing with the air supply line leading from the blower to the end face of the housing and the burner head, in order to minimize the length and depth of the unit.
It is practical to line the end faces of the combustion chamber with fireproof tiles with a labyrinth-like inner structure. These protect the underlying metal parts, isolate the housing from the heat of the flame, and dampen the sound emissions of the burner. It is furthermore appropriate to provide one end face of the housing with a removable cover. It would be advantageous to mount the burner on this cover because this allows easy access to the boiler compartment and the burner.
It is further advisable to use austenitic stainless steel, which resists the aggressive exhaust gas and condensates.